It is extremely difficult to stably disperse carbon black in water at a high concentration due to its hydrophobicity and low wettability with water. This is because the surface of carbon black has an extremely small number of functional groups having a high affinity with water molecules (e.g. hydrophilic hydrogen-containing functional groups such as a carboxyl group and a hydroxyl group). A known measure is to improve the dispersibility of carbon black in water by oxidizing the carbon black to produce hydrophilic functional groups on the surface of the carbon black.
For example, Patent Document 1 (JP-A-48-018186) discloses a method of oxidizing carbon black with a hypohalite aqueous solution, and Patent Document 2 (JP-A-57-159856) discloses a method of oxidizing carbon black by a low-temperature oxygen plasma.
A method of producing an aqueous ink is also known in which the dispersibility of moderately oxidized carbon black in water is improved by using a coupling agent or a surfactant (e.g., JP-A-04-189877). However, it is difficult to stably maintain the dispersibility for a long period of time since the surfactant or the like is oxidized or decomposed due to a change in temperature and a change with time.
Patent Document 4 (JP-A-08-003498) discloses a water-based pigment ink including water and carbon black having a surface active hydrogen content of 1.5 mmol/g or more, and a method of producing a water-based pigment ink including water and carbon black comprising (a) providing acidic carbon black and (b) further oxidizing the acidic carbon black in water using a hypohalite. Patent Document 5 (JP-A-08-319444) discloses a method of producing a water-based pigment ink including finely dispersing carbon black with an oil absorption of 100 ml/100 g or less in an aqueous medium and oxidizing the carbon black using a hypohalite.
According to Patent Documents 4 and 5, a water-based pigment ink with excellent water-dispersibility and long-term dispersion stability is obtained by oxidizing carbon black so that a large amount of active hydrogen (i.e., hydrophilic functional groups) is formed on the surface of the carbon black.
However, the number of hydrophilic functional groups existing at the contact interface between the surfaces of carbon black particles and water molecules is important for dispersing carbon black in water and maintaining a stable dispersed state. Therefore, it is difficult to accurately determine the dispersibility merely by the number of functional groups per unit weight of carbon black. When using a method of finely atomizing carbon black in water using glass beads and oxidizing the carbon black using a hypohalite to treat the surface of the carbon black while improving its dispersibility, the atomization effect is diminished due to the buoyancy applied to the glass beads in water. Moreover, an active site is formed with difficulty. As a result, it is difficult to uniformly form functional groups on the surface of carbon black.
Patent Document 6 (JP-A-11-148027) discloses water-dispersible carbon black which is modified by oxidation and in which the total amount of carboxyl groups and hydroxyl groups among the hydrogen-containing functional groups per unit surface area of the carbon black is 3 μeq/m2 or more.
Patent Document 7 (JP-A-09-286938) discloses a water-based pigment ink in which carbon black is oxidized with a hypohalous acid and/or a hypohalite and at least some of the acidic groups existing on the surface of the oxidized carbon black are bonded to an amine compound to form an ammonium salt.
As the amine compound, ammonia, a volatile amine substituted with an alkyl group having 1 to 3 carbon atoms, an amine compound substituted with an alkanol group (i.e., alkanolamine or alkylalkanolamine), or the like is used. Since amine compounds, with the exception of ammonia, have an extremely low degree of dissociation, a substitution reaction with hydrogen of the acidic group occurs only with difficulty.